Charged particle beam devices are often used to inspect a specimen by detecting secondary charged particles that are generated by a primary charged particle beam interacting with the specimen. If the primary charged particle beam generated by the charged particle beam device is an electron beam, interaction with a specimen typically generates (a) secondary electrons, (b) reflected or backscattered electrons, (c) Auger electrons, (d) transmitted electrons, (e) X-radiation, (f) cathodoluminescence radiation, and/or (g) absorbed (specimen) current.
In many applications, only the secondary electrons, backscattered electrons and Auger electrons are analyzed in order to inspect a specimen. Secondary electrons result from inelastic collisions of the primary electrons with the outer electrons of specimen atoms. As a consequence, the electrons have enough energy to leave the respective shell. Their kinetic energy is in general low. Hence, only electrons close to the specimen surface can escape from the specimen. This is why the analysis of secondary electrons is well suited for specimen surface inspections.
Reflected or backscattered electrons are electrons of the primary beam that have been deflected by collisions with specimen atoms. Their typical energy range extends from the full primary electron energy down to the level of secondary electron energies. Backscattered electrons with a high energy can also be used to inspect a specimen deeper below the surface.
Auger electrons have an energy that is characteristic for the material of the specimen which facilitates an analysis of the material structure of the specimen.
For the following discussion, there is no need to further distinguish between secondary electrons, backscattered electrons and Auger electrons. Therefore, these three types will, for simplicity, be referred to as “secondary electrons”.
Secondary electrons that succeed in leaving the specimen carry information of the specimen by means of their rate of occurrence, by their directions and by their energies. In order to evaluate the information of the secondary beam, a detector for measuring the current of secondary electrons is used. This is shown in FIG. 1, where primary beam 1 generated by source 5 is directed onto specimen 7. The secondary electrons 2 that succeed in leaving the specimen are subsequently detected by detector 6.
For measuring the energy of the secondary electrons, several techniques are known in the art. One technique is described in “Electrical testing for failure analysis: E-beam Testing” by Michel Vallet and Philippe Sardin (Microeletronic Engineering 49 (1999), p. 157-167). Therein, an energy filter grid is arranged between the electron source and the specimen. The energy filter is biased with a small voltage to create a retarding electric field due to which all secondary electrons whose energy is less than a threshold energy will be deflected. Those electrons with higher energy will pass the grid to become detected by a detector.
However, the set-up of M. Vallet and P. Sardin is limited in that it is only a high-pass filter, that is, only electrons with a high energy will pass through the grid arrangement and will be detected. Furthermore, the electric field of the retarding field grid tends to interfere with the primary electron beam 1 in undesirable ways.
Furthermore, another aspect of measuring the secondary electrons should be emphasized: if secondary electrons are analyzed within the framework of object inspection, e.g. wafer inspection, the throughput depends mainly on the four following factors: defect size D, image contrast C, beam current density J and number of columns N working in parallel. As the throughput is proportional to the square of the image contrast C, the image contrast is a very essential factor for improving the inspection throughput.
There are basically three types of contrast: one contrast depending on the secondary charged particles energy, another on the starting angle of the secondary electrons with respect to the incoming beam of primary charged particles and the third on the azimuth starting angle of the secondary electrons. Improvement of each type of contrast will quadratically improve the throughput of an inspection device. A high throughput is indispensable in commercial applications in order to provide for a surface inspection at competitive costs.
It is accordingly an object of the present invention to provide an analyzing system and a charged particle beam device which overcome at least some of the disadvantages known in the art.